The present invention relates generally to papermaking, and relates more specifically to forming fabrics employed in papermaking.
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper xe2x80x9cstockxe2x80x9d) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a xe2x80x9cforming fabric,xe2x80x9d provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the xe2x80x9cmachine sidexe2x80x9d) of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a xe2x80x9cpress felt.xe2x80x9d Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a xe2x80x9cbattxe2x80x9d layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermaker""s fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap or a special foldback on each end, then reweaving these into pin-seamable loops. A number of auto-joiner machines are now commercially available, which for certain fabrics may be used to automate at least part of the joining process. In a flat woven papermaker""s fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction.
In the second basic weaving technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms xe2x80x9cmachine directionxe2x80x9d (MD) and xe2x80x9ccross machine directionxe2x80x9d (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermakers"" fabric on the papermaking machine, and a direction parallel to the fabric surface and traverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term xe2x80x9cendless beltxe2x80x9d as used herein refers to belts made by either method.
Effective sheet and fiber support marking are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Additionally, the forming fabrics should exhibit good stability when they are run at high speeds on the papermaking machines, and preferably are highly permeable to reduce the amount of water retained in the web when it is transferred to the press section of the paper machine. In both tissue and fine paper applications (i.e., paper for use in quality printing, carbonizing, cigarettes, electrical condensers, and like) the papermaking surface comprises a very finely woven or fine wire mesh structure.
Typically, finely woven fabrics such as those used in fine paper and tissue applications include at least some relatively small diameter machine direction or cross machine direction yarns. Regrettably, however, such yarns tend to be delicate, leading to a short surface life for the fabric. Moreover, the use of smaller yarns can also adversely affect the mechanical stability of the fabric (especially in terms of skew resistance, narrowing propensity and stiffness), which may negatively impact both the service life and the performance of the fabric.
To combat these problems associated with fine weave fabrics, multi-layer forming fabrics have been developed with fine-mesh yarns on the paper forming surface to facilitate paper formation and coarser-mesh yarns on the machine contact side to provide strength and durability. For exanple, fabrics have been constructed which employ one set of machine direction yarns which interweave with two sets of cross machine direction yarns to form a fabric having a fine paper forming surface and a more durable machine side surface. These fabrics form part of a class of fabrics which are generally referred to as xe2x80x9cdouble layerxe2x80x9d fabrics. Similarly, fabrics have been constructed which include two sets of machine direction yarns and two sets of cross machine direction yarns that form a fine mesh paperside fabric layer and a separate, coarser machine side fabric layer. In these fabrics, which are part of a class of fabrics generally referred to as xe2x80x9ctriple layerxe2x80x9d fabrics, the two fabric layers are typically bound together by separate stitching yarns. However, they may also be bound together using yarns from one or more of the sets of bottom and top cross machine direction and machine direction yarns. As double and triple layer fabrics include additional sets of yarn as compared to single layer fabrics, these fabrics typically have a higher xe2x80x9ccaliperxe2x80x9d (i.e., they are thicker) than comparable single layer fabrics. An illustrative double layer fabric is shown in U.S. Pat. No. 4,423,755 to Thompson, and illustrative triple layer fabrics are shown in U.S. Pat. No. 4,501,303 to Osterberg, U.S. Pat. No. 5,152,326 to Vohringer, and U.S. Pat. No. 5,437,315 to Ward.
Although these fabrics have generally performed successfully, they have some shortcomings. For instance, various multi-layer fabrics are not auto-joinable with currently available equipment. As noted above, auto-joining refers to an automated process whereby the two ends of a fabric woven in a flat-weaving process are joined to form a fabric woven in a continuous loop. However, auto-joining machines may not be used to join the two ends of a fabric if adjacent machine direction yarns follow the same weave path because the auto-joining machine may not be able to consistently separate such yarns correctly. If the adjacent yarns are paired, then the auto-joining machine may be unable to consistently select the correct yarn during the auto-join process, and instead may select both the correct yarn and the adjacent yarn. Thus, fabrics with paired machine directions yarns in a fabric layer are typically joined by a hand-weaving process, which is more expensive and time consuming than the auto-joining process.
The present invention relates to auto-joinable triple layer papermaker""s forming fabrics which exhibit relatively low caliper values, good mechanical stability, and relatively high permeability.
In one embodiment of the present invention, triple layer papermaker""s forming fabrics having both top and bottom sets of machine direction and cross machine direction yarns are provided in which each yarn in the set of bottom machine directions yarns alternatively pairs with the two yarns in the set of bottom machine direction yarns that are woven immediately adjacent to it. Such fabrics can be constructed so that at least some of the top machine direction yarns interweave with the bottom cross machine direction yarns to bind the top and bottom fabric layers together, or alternatively, may be constructed using a separate set of stitching yarns. If such separate stitching yarns are used, these yarns may be necessary to the formation of the top fabric layer or may be yarns that are separate from, but which weave with, the top fabric layer.
xe2x80x9cIn another embodiment of the present invention, the above-described triple layer forming fabrics may be woven so that each yarn in the set of bottom machine direction yarns weaves beneath half of the bottom cross machine direction yarns in each repeat of the fabric, and/or so that each bottom cross machine direction yarn is woven with the bottom machine direction yarns such that it passes over two adjacent bottom machine direction yarns and passes under the next two bottom machine direction yarns in a repeating pattern. In one specific implementation of this embodiment, half of the yarns in the set of bottom machine direction yarns are woven in a repeating pattern in which they pass over two adjacent bottom cross machine direction yarns, under the next two bottom cross machine direction yarns, over the next bottom cross machine direction yarn, under the next bottom cross machine direction yarn, over the next bottom cross machine direction yarn and under the next bottom cross machine direction yarn. In this implementation, the papermaking surface may be woven in a 1xc3x973 twill pattern.xe2x80x9d
In yet another embodiment of the present invention, triple layer papermaker""s forming fabrics having both top and bottom sets of machine direction and cross machine direction yarns are provided in which each yarn in the set of bottom machine directions yarns is woven with the bottom cross machine direction yarns in a zig-zag pattern. These fabrics may also incorporate a set of stitching cross machine direction yarns that bind the top and bottom fabric layers together, or alternatively may use yarns from the base fabric structures to perform such stitching. In this embodiment, each pair of adjacent yarns in the set of bottom machine direction yarns may be woven so that they together form a repeating series of hourglass patterns in the bottom fabric layer. The papermaking surface of the fabric may be woven in a variety of different weave patterns, specifically including 1xc3x972, 1xc3x973, 1xc3x974, 2xc3x972 and 2xc3x973 twill patterns and a 1xc3x971 plain weave pattern. The bottom machine direction yarns may be woven with the bottom cross machine direction yarns so as to include at least one single float and at least one double float machine direction knuckles on the bottom surface of the bottom fabric layer in each repeat of the fabric, and/or so that the bottom machine direction yarns weave beneath at least half of the bottom cross machine direction yarns.
In another aspect of the present invention, methods of using a triple layer papermaker""s forming fabric for making paper are provided.